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Carrier gas furnace code 34.3: Flame lost after the heating blower started

Gas furnace, Canada and United States

After a successful ignition with the flame proved, the flame was lost after the heating blower on-delay finished.

This needs a licensed gas technician. There are a few safe checks you can do first, marked below. Anything inside the unit is a job for a professional.

This involves the gas supply or burner. Only a licensed technician should open the unit or work on gas parts.

Only some models show this code. Check the list below.

What causes it

InteliSense display furnaces

  1. The gas is off, with the manual shut-off valve to the furnace closed or the control switch on the gas valve turned off.

    Check the gas is turned on to the furnaceYou can do this

    1. Check the manual shut-off valve in the gas pipe leading to the furnace is open.
    2. If the control switch on the furnace's gas valve may be off, use the start-up steps in the owner's manual or on the inside of the main furnace door. Set the thermostat to its lowest setting and OFF, close the manual gas valve, turn off the power and remove the main furnace door.
    3. Turn the control switch on the gas valve to OFF and wait five minutes, then smell for gas, including near the floor.
      If you smell gas, stop. Do not light any appliance or touch electrical switches. Leave the building and call your gas supplier from a neighbor's phone.
    4. If there is no gas smell, turn the control switch to ON, refit the main door, turn the power on and wait one minute, open the manual gas valve, then set the thermostat to HEAT above the room temperature.
    5. If the burners do not light, or the furnace locks out, shut the furnace down and call your dealer.

    Book a licensed gas technician

  2. A dirty flame sensor (oxide build-up) or a flame sensing current that is too low.

    Book a licensed gas technician

  3. Low inlet gas pressure, poor flame carryover or rough ignition, or a faulty hot surface igniter.

    Book a licensed gas technician

  4. A firing rate set wrongly, or a blocked or wrongly gapped carry-over gap.

    Book a licensed gas technician

  5. A grounding problem, such as the green/yellow wire not connected to the furnace sheet metal, poor control ground continuity, or a grounded flame sensor.

    Book a licensed gas technician

  6. A faulty gas valve that does not open when it is powered.

    Book a licensed gas technician

InteliSense ultra-low NOx furnaces

  1. The gas is off, with the manual shut-off valve to the furnace closed or the control switch on the gas valve turned off.

    Same check as above: Check the gas is turned on to the furnace

    Book a licensed gas technician

  2. A dirty flame sensor (oxide build-up) or a flame sensing current that is too low.

    Book a licensed gas technician

  3. Low inlet gas pressure, poor flame carryover or rough ignition, or a faulty hot surface igniter.

    Book a licensed gas technician

  4. A firing rate set wrongly, or a blocked or wrongly gapped carry-over gap.

    Book a licensed gas technician

  5. A grounding problem, such as the green/yellow wire not connected to the furnace sheet metal or the back of the burner box, poor control ground continuity, or a grounded flame sensor.

    Book a licensed gas technician

  6. A faulty gas valve that does not open when it is powered.

    Book a licensed gas technician

How to read this code on your unit

The three-digit display sits on the furnace control behind the blower door. Through the sight glass in the door you see only the status light, which flashes the first two digits of the code. Count the short flashes for the first digit, then the long flashes for the second, so 3 short flashes followed by 3 long flashes is a 33.x code. Write down what you see before you open the door or switch the furnace off. The full code with its third digit shows on the control display only with the door removed and the door switch closed, which is a job for a technician. A technician can also look back at the last 7 faults in the FLt menu. Stored codes are erased by themselves after 72 hours of powered operation after the last fault, and the display shows non when no fault is stored.

Booking a licensed gas technician

Book a licensed gas technicianlicensed gas technician

  1. Turn the unit off and leave the covers on. Gas components must only be serviced by a licensed technician.
  2. Note the code, how often it appears, and the model and serial numbers from the rating label.
  3. Book a licensed gas technician and give them those details.

Models that show this code

102 models in 9 ranges
RangeModels
58SB0B/58SB1B58SB0B045M14-12, 58SB0B045M17-14, 58SB0B070M14-12, 58SB0B070M17-12, 58SB0B070M17-16, 58SB0B070M21-16, 58SB0B090M17-14, 58SB0B090M21-16, 58SB0B090M21-20, 58SB0B090M24-20, 58SB0B110M21-20, 58SB0B110M24-20, 58SB0B135M24-20, 58SB0B155M24-20, 58SB1B045M14-12, 58SB1B045M17-14, 58SB1B070M14-12, 58SB1B070M17-12, 58SB1B070M17-16, 58SB1B070M21-16, 58SB1B090M17-14, 58SB1B090M21-16, 58SB1B090M21-20, 58SB1B090M24-20, 58SB1B110M21-20, 58SB1B110M24-20, 58SB1B135M24-20, 58SB1B155M24-20
58SC0B/58SC1B58SC0B045M14-12, 58SC0B045M17-14, 58SC0B070M14-12, 58SC0B070M17-12, 58SC0B070M17-16, 58SC0B070M21-16, 58SC0B090M17-14, 58SC0B090M21-16, 58SC0B090M21-20, 58SC0B110M21-20, 58SC0B110M24-20, 58SC0B135M24-20, 58SC1B045M14-12, 58SC1B045M17-14, 58SC1B070M14-12, 58SC1B070M17-12, 58SC1B070M17-16, 58SC1B070M21-16, 58SC1B090M17-14, 58SC1B090M21-16, 58SC1B090M21-20, 58SC1B110M21-20, 58SC1B110M24-20, 58SC1B135M24-20
58SP0B/58SP1B58SP0B045V14-12, 58SP0B070V14-12, 58SP0B070V17-16, 58SP0B090V17-16, 58SP0B090V21-20, 58SP0B110V21-22, 58SP0B135V24-22, 58SP1B045V14-12, 58SP1B070V14-12, 58SP1B070V17-16, 58SP1B090V17-16, 58SP1B090V21-20, 58SP1B110V21-22, 58SP1B135V24-22
59SC2E59SC2E040M14-12, 59SC2E040M17-12, 59SC2E060M14-12, 59SC2E060M17-14, 59SC2E080M17-16, 59SC2E080M21-20, 59SC2E100M21-20, 59SC2E100M21-22, 59SC2E120M24-20
59SC6A59SC6A026M14-10, 59SC6A040M14-10, 59SC6A040M17-12, 59SC6A060M14-12, 59SC6A060M17-16, 59SC6A080M17-16, 59SC6A080M21-20, 59SC6A100M21-20, 59SC6A100M21-22, 59SC6A120M24-22, 59SC6A140M24-22
58CU0B58CU0B040C17-12, 58CU0B060C17-16, 58CU0B080C21-20, 58CU0B100C21-20
58SU0B58SU0B040M17-12, 58SU0B060M17-16, 58SU0B080M21-20, 58SU0B100M21-20
59CU5B59CU5B060C17-16, 59CU5B080C21-20, 59CU5B100C21-22
59SU5B59SU5B040M17-12, 59SU5B060M17-16, 59SU5B080M21-20, 59SU5B100M21-20, 59SU5B100M21-22

Where this comes from

Checked against the manufacturer's manual on 11 October 2026.

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Other Carrier gas furnace codes

Code Meaning What you can do
34 Flame not proved during ignition Safe checks first, then a pro
14.2 Flame lost 3 times after 70 seconds of heating Safe checks first, then a pro
14.3 Lockout after 7 flame losses in one heat request Safe checks first, then a pro
33.1 Limit circuit switch open, blower running to cool the furnace Safe checks first, then a pro
33.2 High heat only for the current cycle Safe checks first, then a pro
34.1 Ignition fault, burners did not light in 4 trials Safe checks first, then a pro
34.2 Flame lost before the heating blower started Safe checks first, then a pro
34.4 Flame lost at reduced inducer pressure or in Self Healing mode Safe checks first, then a pro
34.5 Low heat flame lost before the blower started Safe checks first, then a pro
34.6 Low heat flame lost after the blower started Safe checks first, then a pro
34.7 High heat flame lost before the blower started Safe checks first, then a pro
34.8 High heat flame lost after the blower started Safe checks first, then a pro

See all flame loss codes on gas furnaces across brands.